In high temperature application, crystalline materials often have superior resistance to thermal deformation compared to their glassy counterparts. Crystalline articles are often produced by sintering. Sintering has certain disadvantages, including void creation and the need for pressing. In contrast, glassy articles may be cast in a nearly void-free state. Unfortunately, glassy materials can thermally deform well below their theoretical melting point.
Prior art teaches that crystalline grains can be annealed from an initially glassy material, thereby forming a glass-ceramic. Glass-ceramics can have improved resistance to thermal deformation compared to the glass. The glass is first shaped, usually above the liquidus temperature, and then annealed above a nucleation temperature. The fluid nature of the glass above its liquidus can permit it to be shaped by casting. The nucleation temperature is a temperature at which crystals begin to form and grow within the glass. The selected nucleation temperature should be below a temperature at which the glass would thermally deform. During annealing, crystalline grains nucleate, begin to grow, and ultimately comprise a majority of the material. High melting point compounds, such as titanium dioxide, can facilitate nucleation. Below its melting temperature, a crystal typically has significantly lower thermal deformation than a corresponding glass. Glass-ceramics have been used in a variety of applications where resistance to thermal deformation is an issue.
One such glass-ceramic composition includes a predominately crystalline phase comprising cordierite. Cordierite, a magnesium aluminum silicate, was described by S. D. Stookey in U.S. Pat. No. 2,920,971, and may be used in refractory applications. Cordierite glass-ceramics have good hardness and resistance to thermal deformation, but they can suffer from a high coefficient of thermal expansion (CTE) and have only average fracture toughness. For example, one cordierite glass-ceramic has a fracture toughness of 2.2 MPa·m0.5 and an average CTE of 57×10−7/° C. over the temperature range from 25-1000° C. Poor fracture toughness permits cracks to form and propagate, which can cause an article to shatter or break under stress. A high CTE decreases resistance to thermal shock. Low fracture toughness and high CTE limit the utility of cordierite glass-ceramics.
Silicon nitride, Si3N4, has been used in applications requiring lower CTE and higher fracture toughness. Silicon nitride has a CTE of about 30×10−7/° C. and a fracture toughness around 6 MPa·m0.5. Silicon nitride also has better high temperature capabilities than most metals combining high strength, creep resistance, and oxidation resistance. These properties have allowed silicon nitride to replace metals in turbine and reciprocating engines, and as engine components, bearings and cutting tools. In addition, its low thermal expansion coefficient provides good thermal shock resistance compared with most ceramic materials. Negatively, silicon nitride can be difficult to produce as a fully dense material (often requiring hot pressing), does not readily sinter, may oxidize under certain conditions, and cannot be heated over 1850° C. because it dissociates into silicon and nitrogen. These deficiencies cause silicon nitride components to be expensive, thereby limiting the applications using silicon nitride components.
A need exists for a replacement material to silicon nitride. The material should be relatively inexpensive and easy to produce. Preferably, it should be amenable to molding and should combine low CTE with high fracture toughness.